Ultrasound technology is a widely used technique for nondestructive testing of various structures such as the hulls of vehicles and pipelines. Particularly, Lamb waves have been investigated as a type of guided ultrasound waves for investigating the structural state of plate-like, or thin-wall, structures. As is generally known, Lamb waves are elastic perturbations that propagate in a solid plate with free boundaries. During propagation through a solid plate structure, Lamb waves form several symmetric and anti-symmetric modes according to the plate thickness and acoustic frequency of the waves specified by their phase velocities. One advantage of Lamb waves is that they are able to propagate long distances in plate structures. Moreover, in contrast to conventional ultrasonic methods, where inspection of the structure is conducted point-by-point, a line is inspected at each position of a transducer. Therefore, Lamb waves may save significant time when investigating large structures.
Although ultrasound is an excellent technique for detecting structural flaws, it may be relatively inefficient when it comes to evaluating flaw size, shape, and orientation. The spatial resolution and signal to noise ratio of the resulting ultrasound image may be improved by additional numerical processing of the ultrasound data. For example, when a collection of sensors are used, the ultrasonic wave may be steered and focused to enhance the resolution and sensitivity of defect detection over an inspected region. Such methods hold great potential for implementing structural health monitoring (SHM) systems and pursuing prognostics and remaining life approaches to asset management.
Various methods for steering and focusing ultrasonic Lamb waves exist. One of these methods is known as Embedded Ultrasonic Structure Radar (EUSR). The EUSR method is one of the oldest and simplest delay-and-sum beamforming approaches. EUSR assumes that data from an N-element sensor array is collected in a “round-robin” fashion, using one sensor element at a time as a transmitter and the remaining sensor elements as receivers (either including or excluding the transmitting element). Then, with a total of N×N (or N×N−1) data signals, EUSR implements beamforming as a signal post-processing operation.
Conventionally, EUSR is performed in the time domain. As such, it does not account for the dispersion of Lamb waves. That is, conventional EUSR does not account for the dependence of ultrasonic wave velocity on the plate thickness/pulse frequency product. This dispersion results in a spatial resolution degradation of the resulting EUSR image, which decreases the accuracy (both location and size) of damage detection. One widely used approach to mitigate the impact of dispersion is to drive ultrasonic sensors using narrowband excitation pulses with center frequencies tuned to the low dispersive region for particular Lamb wave mode. However, one drawback of this approach is the limitation of pulse frequency to low dispersive part of the dispersion curve which may not be optimal for a particular application (e.g., material or defect type).
Another approach attempts to compensate for the impact of the dispersion in the input data (i.e., raw transducer data) prior to EUSR processing. Such algorithms attempt to compensate for the elongation of an ultrasonic pulse when propagating in dispersive material, thereby resulting in spatial resolution in the direction of the beam propagation. An improvement of the spatial resolution in the direction of beam propagation may be observed, though no effect in the perpendicular direction is clearly demonstrated.
Accordingly, it is desirable to provide an improved method and system for implementing ultrasound for use in the detection of structural flaws. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.